A manufacturing operation often entails the use of hoists to manipulate items. In a semiconductor wafer fabrication line, hoists are used to lift wafer stock during various phases of the wafer fabrication process. Such a hoist is typically mounted on an overhead rail system, allowing hoisted items to be moved around the facility. Items are raised and lowered by such a hoist onto processing stations, where various wafer treatment operations are performed.
One type of hoist system has three roller assemblies in a triangular orientation with a wide suspending member, or band, attached to a rotating drum. See, for example, U.S. Pat. No. 5,673,804, issued Oct. 7, 1997, entitled Hoist System Having Triangular Tension Members, and assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. In this type of hoist, items are hoisted by rotating the drums such that the band is wrapped around it, drawing objects upwards.
For this type of system to operate effectively the band must be wound such that its centerline is wrapped up over itself consistently with each revolution of the drum. For a band to wrap onto a drum in this manner, the band must be mounted to the drum so that the center line remains perpendicular to the drum. Tension in the band must therefore also be maintained perpendicular to the drum. Since the load being carried by the hoist is suspended from the free end of the band, the band tends toward a vertical orientation. A line of force along the center of the band defines the lifting force and likewise has a tendency to remain vertical due to gravity. The drum must therefore be oriented horizontal, perpendicular to the band, which requires that the hoist housing in which the drum is mounted also be horizontal. However, the overhead rail system that a hoist is suspended from is not an unyielding structure, therefore it cannot always maintain the hoist housing at a precise horizontal orientation. Further, factors such as air currents in the manufacturing facility, imperfections in the band material, and friction between the band and drum can also contribute to the deviation of the load from vertical. Such deviation creates a tendency for the band to "walk" off the end of the drum, causing it to bunch up at one end of the roller and resulting in an uneven load, increased band wear, and even failure to unroll and lower the hoisted object.
Such hoisting operations are further challenged by the requirement of a substantially contaminant free environment required for wafer fabrication operations. Particulate contaminants, such as dust, dirt and other particles, must be minimized. Therefore, moving parts must be constructed of materials resistant to physical abrasion and deterioration. Lubricants between moving parts must also be carefully chosen to avoid aggravating the presence of foreign material. For this reason, high band ridges on the drum edge which steer the band by frictional forces are undesirable.
It would be beneficial to develop a roller assembly which keeps the band centered on the drum as it is rolled without increasing friction, abrasion, and lubricant usage which compromise the substantially contaminant-free environment required for wafer processing.